The mechanisms of sorption of four steroidal hormones - estradiol, estrone, progesterone and testosterone - to electrodialysis (ED) membranes were investigated as a function of solution pH and presence of humic acid (HA). Hormone-membrane partition coefficients (log K(AEM/CEM)) determined through sorption isotherm experiments suggested that hormone sorption was due to hydrogen bonding and cation-pi interactions between hormone and membrane functional groups. Progesterone sorption at pH 7 (922 mu g/cm(3)) during ED was greater than estrone sorption (591 mu g/cm(3)) due to its greater cation-exchange membrane (CEM) bonding affinity. Estrone sorption at pH 11 (487 mu g/cm(3)) was reduced due to estrone dissociation and electrostatic repulsion with negatively charged CEMs. Permeation of estrone (30-100 ng/cm(2) h) through the anion-exchange membranes (AEMs) was observed. At pH 11, charge repulsion between estrone and HA coupled with AEM electrostatic attraction resulted in increased sorption. Partial membrane desorption was noted in isotherm (20-30%) and ED desorption (3.8%) experiments and was dependent on the initial mass sorbed, solution pH and resultant electrostatic interactions. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.